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Abstract

Laser Speckle Contrast Imaging (LSCI) has become a widely used technique to image cerebral blood flow in vivo. However, the quantitative accuracy of blood flow changes measured through the thin skull has not been investigated thoroughly. We recently developed a new Multi Exposure Speckle Imaging (MESI) technique to image blood flow while accounting for the effect of scattering from static tissue elements. In this paper we present the first in vivo demonstration of the MESI technique. The MESI technique was used to image the blood flow changes in a mouse cortex following photothrombotic occlusion of the middle cerebral artery. The Multi Exposure Speckle Imaging technique was found to accurately estimate flow changes due to ischemia in mice brains in vivo. These estimates of these flow changes were found to be unaffected by scattering from thinned skull.

(a) Illustration of partial craniotomy model. The regions enclosed by the closed loops (regions 1,3 & 5) are located in the craniotomy. Regions outside of the closed loops (regions 2,4 & 6) are located in the thinned (but intact) skull. (b) Time integrated speckle variance curves illustrating the influence of static scattering due to the presence of the thinned skull. A decrease in the value of ρ indicates an increase in the amount of static scattering. Regions 2 and 4 show distinct offset at large exposure durations. This offset it due to increased νs over the thinned skull

Relative blood flow change caused due to the ischemic stroke in the branch of the MCA (Region 1 in Figure 4a). (a) Time course of relative blood flow change in Region 1 as estimated using the MESI technique. The flow estimates in first 10 minutes were considered as baseline. The reduction in blood flow due to the stroke, is estimated to be ~ 100%, which indicates that blood supply to the artery has been completely shut off. (b) MESI curves illustrating the change in the shape of the curve as blood flow decreases. The MESI curve obtained after the stroke is found to be similar in shape to that obtained after the animal was sacrificed. This is a qualitative validation of ~ 100% decrease in blood flow in the artery

MESI technique can predict consistent blood flow changes across the thin skull — craniotomy boundary. (a) Relative blood flow changes estimated using the MESI technique in 3 pairs of regions across the boundary (Figure 4a). The change in blood flow is found to be similar for each pair of regions. (b) Relative blood flow changes estimated using the LSCI technique (at 5ms exposure) in 3 pairs of regions across the boundary (Figure 4a). The change in blood flow is not similar for each pair of regions. This difference is especially prominent over the vessel (Regions 1 and 2)

Full field relative correlation time maps obtained using the (a) MESI technique (b) LSCI technique (5ms exposure). Three corresponding regions marked in the figures illustrate the superior performance of the MESI technique. The boundary (corresponding to the boundary between the thin skull and the craniotomy) indicated by the red arrow is clearly visible in (b), but not in (a). There is a clear change gradient in the region indicated by the star in (b), but this gradient is invisible in (a). The vessel circled is more visible in (a) compared to (b). Relative correlation time estimates obtained using the MESI technique are not affected by the presence of thinned skull. Hence, similar estimates of blood flow changes are obtained across the boundary between the thin skull and craniotomy regions, leading to the absence of any gradients in blood flow change in (a)

Comparison of the percentage reduction in blood flow obtained in regions 1 and 2 (Figure 4a) using the MESI technique using two different speckle expressions (Lorentzian: Equation 1 and Gaussian: Equation 2) and multiple single exposure LSCI estimates. MESI estimates are found to be more consistent in estimating blood flow decreases across the boundary between the thin skull and the craniotomy. There is significant difference between the two models in estimating blood flow decrease